The increasing requirement of rapid assembly in assembly plants such as manufacturing plants has resulted in the development of sophisticated assembly tools. For example, with regard to the tightening of joints, a threaded fastener such as a nut, screw or bolt often has to be rotated a number of turns with a relatively low torque prior to the fastener reaching a point where the joint actually starts to tighten and the torque thereby starts to rise.
Consequently, it is highly desirable that the initial threadening or running down phase can be carried out as quickly as possible, since this initial number of turns often is considerably greater than the number of turns (or even part of a turn) that the fastener rotates during the actual tightening phase, and since otherwise a considerable portion of the total assembly time of a particular joint can be consumed during the initial stage of threading.
For this reason, electrically powered assembly tools have been developed where the tightening of a joint is carried out in two steps, namely a first step during which the joint is tightened at a high speed to a predetermined torque level, whereafter the joint is further tightened up to a final predetermined pretension level in a second step at a lower speed.
However, such tools can, in particular with regard to high-torque joints (e.g., in the order of 50 Nm or more), impose undesired jerks of the tool when the torque starts to rise if the operator is unprepared to the sudden torque increase. Such jerks can be very uncomfortable to the operator, and also be a risk of danger if the operator is subject to a powerful jerk of the tool, e.g. when standing close to a wall or sharp objects.
Therefore, control methods have been developed, where the rotation speed of the tightening tool in the second step is controlled in a manner such that it is possible to obtain a tightening process that is not only fast, but which is also more advantageous from an ergonomic point of view.
According to the prior art there basically exists two methods of accomplishing the tightening of threaded joints, both being two-step tightening methods where the first method essentially starts with a high, substantially constant rotational speed until the tightening torque has reached a threshold, whereafter a pause is imposed to prepare the operator for the subsequent torque increase that is about to come. In the second step, the threading is operated at a reduced speed and is kept constant until the tightening torque has reached its target level.
The second method is in fact a one-step method and comprises a first phase that is similar and rather “static” to the above, but wherein in a second phase, instead of first reducing the speed to zero as above, the speed is immediately reduced to an intermediate speed which then keeps the tightening speed constant until the target torque has been reached.
Although the above described methods are capable of providing a substantial improvement for the operator from an ergonomics point of view, and to a great extent reduce the tiring and uncomfortable jerks that normally occur during a tightening process, the tightening process will still remain similar for all operators, with the result that while the above tightening processes can be perceived as comfortable to some operators, the tightening process can be perceived as having too low degree of flexibility for others.
Consequently, there exists a need for an improved electric screw joint tightening tool that is capable of being operated by means of more flexible methods to thereby improve ergonomics and operator satisfaction.